Human Intestinal Parasites – Causes, Symptoms, Treatment, Prevention

What Are Intestinal Parasites?

The term parasite refers to organisms that infest another organism where it lives and feeds. Some of these parasites are microscopic and not visible to the naked eye whereas other parasites are relatively large. Human intestinal parasites are either one-cell organisms or intestinal worms that live in the small or large intestine and consume nutrients from the intestinal lumen or the blood flowing in the intestinal wall.

One-Cell Parasites – Protozoa

One-cell organisms, like Giardia lamblia, also called Giardia duodenale (Picture 1), Cryptosporidium (crypto) and Cyclospora, utilize nutrients from the stool. They belong to a living kingdom Protozoa (Gk. protos = first; zoa = animals). They may cause inflammation of the small intestine thus hampering absorption of nutrients. Entamoeba hystolytica lives predominantly in the colon.

Giardia under electron microscope

Picture 1. Parasite giardia from a stool sample,
as seen under the electron microscope (actual size:~0.01 mm)

Intestinal Worms (Helminths)

Intestinal worms (helminths), like roundworms (hookworms, whipworms, Ascaris and Trichinella), tapeworms and flukes, are few millimeters to several meters in size. These worms feed from the contents of the bowels or suck the blood from the intestinal wall and can cause about the similar symptoms as one-cell parasites. However, in most instances these intestinal worms cause little to no symptoms for months or years.

How Can One Contract Parasites?

Both protozoan and helminths (worms) are primarily contracted through ingesting contaminated food or water. The adult parasite or its eggs are shed in the stool of humans or animals. This can then contaminate water sources suahc as rivers and lakes as is the case with open sewage or poor water treatment practices. Drinking, bathing or irrigating crops with this contaminated water then allows the parasites to enter the body.

In some instances, parasites can be spread from person to person mainly when one person comes into contact with the fecal particles of a person who already has an infestation. This puts certain people at a greater risk. For example a caregiver changing diapers or cleaning soiled lined  of a person who already has an infestation or infection may therefore be at risk of contracting the infection.

Symptoms of Intestinal Parasites

Intestinal parasites can cause one or more of the following symptoms:

  • Mild diarrhea with waxing and waning course, appearing few days to several weeks after the travel into the country with poor hygiene, and lasting from several days to months (for comparison: bacterial diarrhea usually has a sudden and dramatic onset within some hours after a meal, and heals on its own in few days).
  • Mucus in the bowel movement
  • Bloating and flatulence
  • Weight loss is common.
  • Pale skin, tiredness, tingling (due to reduced absorption of minerals and vitamins).
  • Entamoeba histolytica may cause severe colitis with ulcers, abdominal cramps, blood in the stool with occasional pus – the disease is known as amebic dysentery (9). Entamoeba may invade liver, lung, brain, or other organs, where it forms cysts.
  • Worms, their parts or eggs can be sometimes found in the stool. Read more about intestinal worms.
  • Infection by either one-cell organisms or intestinal worms is often present without any symptoms.

Diagnosis of Intestinal Parasites

Diagnosis can be often suspected from history of prolonged bloating or diarrhea and can be confirmed by the following tests:

  • Ova and parasites (O&P) test of the stool. The test is often false negative so three stools from three different days (at least two days apart) should be tested.
  • Hemoccult test may reveal blood in the stool
  • Blood tests often reveal elevated eosinophils and IgE antibodies and lowered ferritin, hemoglobin or red cells
  • CT or biopsy are needed to find cysts in the liver, lungs or brain.

Treatment of Intestinal Parasites

Anti-parasitic drugs, like metronidazole, quinacrine, tinidazole and furazolidone are usually effective, but the exact treatment regime, as prescribed by your doctor, should be followed.

Paramomycin is not absorbed from the intestine into the blood, so it may be used in pregnancy (1). For cyclospora trimethoprim-sulfametoxazol can be used (2).

Intestinal parasites can be successfully eradicated, but may recur if the source of infection still exists. Delayed treatment can lead to a host of complications, some of which can be life threatening, as may be the case of severe dehydration in diseases like giardiasis.

Prevention of a Parasitic Infection

An infection by intestinal parasites can be prevented by:

  1. Hand washing with an antiseptic soap before eating and after using the toilet, particularly after a bowel movement, is imperative in preventing infections.
  2. Cooking food at high temperatures kills all parasites. Partially cooked meat or raw meat are high risk foods for passing on these parasites. Smoking or freezing does not destroy the parasites or eggs.
  3. Washing raw vegetables and fruits with a suitable disinfectant solution that is fit for consumption. Salt water may also be helpful to some extent. Clean running water should be used for rinsing.
  4. Consuming safe food and water is important to prevent infections. Travelers from western countries visiting Central or South America, Africa or South Asia should ingest only packaged, bottled or cooked foods and drinks and only fruits that they can peel by themselves.

Read more on food and water safety tips.

For How Long Is a Person with Parasitic Diarrhea Contagious?

Most parasites are transmitted through contact with the person’s feces and to a lesser extent with body fluids. It is usually not airborne so transmission cannot occur with droplets that become airborne from coughing or sneezing. A person may be infectious for prolonged periods of time, depending on the causative parasite.

Usually a person is contagious, or more specifically the stool contains the parasite, a short while after contracting the infection and for as long as the infection is active. Even without any symptoms being present a person may still be contagious. With some parasites like Entamoeba hystolitica a person can be contagious for months and even years thereafter.

Related Articles:


  1. Incidence of intestinal parasites in homosexual men (
  2. Intestinal protozoa (
About Jan Modric (249 Articles)
Health writer

Please note that any information or feedback on this website is not intended to replace a consultation with a health care professional and will not constitute a medical diagnosis. By using this website and the comment service you agree to abide by the comment terms and conditions as outlined on this page

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  • Hi PS. This could be fiber since the human body cannot digest fiber. However, this stringy substance may be mucus or even worms as well. There is no way of saying for sure. You need to see a doctor who may consider further investigations like a stool analysis. If this is mucus and in light of the anal itching, this could be symptoms of hemorrhoids. However, it needs to be diagnosed by a doctor.

  • Heather

    I was wondering if you could tell me what kind of intestinal parasite this is? It was found in a stool sample.

  • Audrey Neitling

    For the past week I have been passing this awful stuff along with my stools. I thought maybe roundworms. The first few days I didn’t get pictures but I can describe it as looking like a wad of alfalfa sprouts about the size of a large strawberry. Also, I suffer from gas, bloating, very loose, informed and mucous stools. So I emailed these pics to an online parasite doc who said definitely tapeworms. Do you agree? I am making an appointment with my own doctor in Monday.

  • Hi Audrey. We have not seen the pictures and either way we would not diagnose you online as it against our terms of service. Rather speak to your doctor about your concerns. A stool sample may be necessary which will then be sent to a laboratory. If there are worms, or segments of it, or the eggs in the stool then your doctor will be able to reach a diagnosis and prescribe the appropriate medication. Many of your symptoms may be indicative of an intestinal worm infection. However, there are instances where long string objects in the stool are actually mucus and mistaken for worms. Mucus in the stool can arise for may reasons but with some of the digestive symptoms that you have mentioned it could be conditions like irritable bowel syndrome or inflammatory bowel disease.

  • Christina

    My husband has been having problems with his stomach. He has been bloated and has a mild case of diarrhea. He said he vomited and a worm came out about 3 inches long. Then again in his stool. He has tried at least 2 different medications but he still has the bloating and a little irritation in his stomach. After one of the medications he passed the one in his stool.
    Now he has had 2 nose bleeds and he’s gone to doctors but they only prescribed him those medications.
    Is there a way to know what kind of parasite he has and what is best to get rid of them for good?

  • Hi Christina. Yes, he could have a stool test done. A sample of stool would be sent to a lab where it would be analyzed. Even if there are no worms in it, they should find the tiny eggs of these worms which would verify the diagnosis. It is advisable that he has a stool test done. If necessary he should consider consulting with a gastroenterologist since his symptoms are persisting. Remember that eradication of parasitic intestinal worms doesn’t occur overnight and sometimes the medication may need to be taken again in a repeat dose.

  • nothingac

    um i have worms in my poo and how do you get worms and what do they do to you

  • william

    I have been experiencing an itchy anus for at least 8 months to a year if not longer. And in the past 6 months I have felt something moving or squirming inside around my anus. I also have developed a number of unexplained white spots on my face and a couple on my chest. A dermatologist was confident they are Biers spots but they have continued to show up and I can’t help but think that they are related to the squirming sensation inside my rectum. There haven’t been any worms in my feces or anything, but I have found myself more gassy and there is ocasionally a mucus like substance when I wipe? A couple times in the past 6 months I have noticed a some blood looking substance attached to my feces. I took one full cycle of combantrin to try to flush out the potential worms to no avail. Please help!

  • Aylin

    Hello there, please help !

    3 weeks ago I was back from a holiday in Hong Kong. I am living in Beijing, China, but I am a European people from East Eurpe. In Hong Kong, exactly after I arrived, I was check in hotel and went to the beach. I was swimming in the ocean then I want5ed to back to my hotel. Instantly , in the bus I started feeling so bad, dizzy, nausea and I was vomiting and felt as I was never feel before. I arrived to the hotel at least, where the symtoms continued and second day I had diarheea and next day the same, after every meal, 30 mins later I should be near a toilet or other way….. Continued nausea and unclear vision and dizzy. I put all this on my ovulation period , but in Beijing I had never this bad symtoms, just mild. I was thinking is something from ocean, a “sea” bad feeling or something or the climate something. I back to Beijing in one week later and felt better. This symtoms disappeared, but almost 3 weeks later, I developed something in my left feet , like a cyst, like a vain , or inflamation , painful and which not pass away with any kind of medication like antiinflamatory or other pain killers. I started an antibiotic Levofloxacin , also without effect after 5 days.. 2 days later, there appeared other “inflamation” or cyst in my upper back , feft side, the same painful, persistent pain and without any effect to any kind of medication or gel for massage as Flagyl. I was taken a dose of Tinidazole and next day I noticed the pain from my foot was little smaller and the inflamation not so big as a day before. But I continue to have joints pains , suddenly came and go and then it’s come back in other part of body or the same joint or other muscle of the body as in the left side today, but lumbar area muscle. I am still on Levofloxacin to end the course and I also continue with 2 more dose of Tinidazole. I am wonder if Albendazole is the same thing as Tinidazole or have a larger area of coverage ? Am I on the right treatment ? What hospital section should I go with all this ? I also don’t know if my blood tests will be influenced by the medication I am curently on. Is this possible ? If I go to neuro-surgeon, or orthopedy, they will treat me for atritis probably, which is not the case ! I do admit I have bones problems and pain in autumn , but nothing on this kind, there was only pains, but nothing was grow up in muscles and feet termination ever ! And usually , when I had a bones pain due to clmate chnage or pressure, I was ok with antiinflamatory pills, but now no have any effect and is not related to this. In the feel and back is like a tumor, a vain inflamed or a cyst . Is not visible, only when I touch it and press it can be feel and when I am walking is become painful. All the back muscles now I feel painful. I mantion that I am also on Serrapeptase enzime tretament since 3 months, for Endometriosis scar tissue . It helped me a lot with the menstrual problems and pains, but I read several comments also about this enzime that gives such reaction in some peoples. Is this true ? Have you any more idea ?
    Please help me where to go ? which kind of doctor and what kind of tests to ask to be done ? I am foreigner here and difficult to talk with doctors as they don’t speak English well or not speak at all.
    Thank you !

  • Amy

    Hello, I was diagnosed with parasites but the doctor didn’t really tell me which parasite, he prescribed tinidazole for two days with doses of 1g every 12 hours. I’ve already taken 3 out of the 4 dozes but i’m still feeling nauseated, the only thing that has changed is that my stool it’s not as liquid as before but it’s still kind of liquid also i’m not going to the bathroom as much and i’m still seeing mucus on my stool. Is this treatment working? Or should I go back to the doctor’s office.

  • Brandon Lease

    HI, my name is Brandon.
    my dad and I contracted what we think is a fluke, my dad and I have both accumulated over 40 bumps ranging in size all over our body, legs, back near kidney, head, arms, chest, and stomach. About five years ago we went to a wedding in Mexico, the tropical water was warm and inviting. we decided to go snorkeling. Upon entering and swimming out, we found the water was dirt and filled with salt, we could see the water as it moved. So we got out and left, we think we were near a sewage drain or something. But anyways, we came home and we felt sick, and tired. and one day we both actually couldn’t function we were so sick. The sickness passed but we started getting lumps, and we were still really tired, and I actually was losing muscle mass. Back in high school I was benching 245 pounds, and I was slowly getting weaker, I can no longer bench without hurting my spine. I got really sick just recently and was diagnosed with CDIF and my stomach pain is so bad it is hard to eat. I went to the doctor and they got me feeling kind of normal but the stomach pain continues, and im still getting lumps, I know, because when I get them it’s like I have a needle hanging off wherever the bump is. also my joints are really weak… every bone can pop ;/ My dad went to UW twice they told him he was crazy and self diagnosing, and that he was going to have to just cut out the lumps as they appear. I’m not willing to waist anymore of my life with these things in me, and I don’t know a doctor that works with parasites around me. I live in the Seattle area, my dad and I really need help!